Non working serial port interfaces in software that fails with USB to Serial Converter cables is often not the fault of the USB serial device or its OS drivers.

I would venture to suggest that this is "usually" the case - not just "often"...?

Many old time programs written for use with the serial ports in a PC were written with direct access to the COM port I/O addresses and interrupt vectors.

Not only that, but many (most?) were also abusing the RS232 control lines as GPIO.

This would generally fail even on a "real" serial port under modern Windows versions.

There are a slew of these old programs still in use even today and is one of the reasons that Microsoft in particular has mapped the COM port register addresses as still accessible from user program space!

But the timing is entirely different!

Newer and more smartly written software that uses the OS API to open and access serial port devices, whether they be the old legacy COM1 to COM4 hardware supported devices, virtual COM ports mapped through to USB serial cables or virtual ports offered by drivers ... stand a much better chance of working properly with non legacy hardware.

Absolutely.

Note also that some "USB-to-Serial" chips do have a "GPIO" mode (some of FTDI's certainly do); but this requires a special driver & API - not just the bog-standard COM port...